This invention relates to a control system for a gas turbine engine and, more particularly, to an integrated control system including a primary control and a back-up control, the latter being adapted to provide continued operation of the gas turbine engine in the event of malfunction or failure of the primary engine control system.
Full authority electronic control systems for gas turbine engines provide a number of significant advantages over conventional hydromechanical control systems. However, since full authority electronic control systems are only advancing to what might be termed a first generation level, confidence in the reliability of such electronic controls has not attained the level generally associated with the hydromechanical systems which have been in use for many years in highly refined configurations. Consequently, it is advisable to complement the electronic control system with a secondary or back-up hydromechanical control system which will assure continued operation of the engine in the event the primary electrical control system exhibits a malfunction or a failure.
In a gas turbine control system having back-up capability, it is necessary to have compatibility between the primary electronic control system and the back-up hydromechanical control. In other words, the back-up system must not interfere with the primary system when the latter is in control of the engine and vice versa. Prior art control systems have approached the compatibility requirement by rendering the back-up control entirely inoperative while the engine is under the control of the primary control system. Transfer from one control system to the other is accomplished either by a manual switch operated by the operator or by automatic means. In either case, however, a period of time is encountered wherein uncontrolled transients may be introduced into the control of the engine as the primary system relinquishes control of the engine and the back-up system assumes such control. Additionally, it is usual practice to design the primary control and the back-up control upon different control theory whereby switching from one system to the other may result in step-type changes causing an immediate alteration in the operation of the engine. Transients and step-type changes in engine operation may require immediate operator attention and are therefore undesirable.
Another problem exhibited by prior art control systems having primary and back-up capability is associated with inactivity of the back-up system while the primary control is in operation. Since extended periods of time, usually years, may pass while the primary control is fully operable, the back-up system may deteriorate, unknownst to the engine operators, to a condition wherein it will not function properly when called upon in the event the primary system fails.